It means that the application you are working with is making too many calls to the API in a short period of time, and it will need to adjusted to send requests in a measured manner.
The application can make up to 20 API request calls in 5 seconds, or 20 requests in 10 seconds. In either case, the application will have to wait 10 seconds before any further API calls can be made from your public IP address. If either limit is reached, the "Error 429 TOO MANY REQUESTS" error will be returned on the response from the API.
If you see that your application is not making this many requests to Pressero, you may want to check that your account is not also running the Automated Workflow Integrator in addition to the API. That could be causing more requests than expected to happen from your public IP address.